[avalanche] Allow to manually set the session key

Summary: This makes debugging easier.

Test Plan:

ninja all check-all

Run the node with a custom session key and check it works.

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D8572